Conventionally, various kinds of methods have been proposed and realized as a method for achieving high-speed and high-quality wireless transmission of a large bulk of information such as image information, etc. For example, according to a CDMA scheme, transmission data is subjected to spread processing by using a spreading code corresponding to each communications terminal for transmission thereof. In the CDMA scheme, this makes it possible to reduce interferences between transmission signals on wireless propagation paths, thereby making it further possible to obtain high-quality reception signals at receiver sides.
Recently, an OFDM-CDMA scheme, which is a combination of an OFDM modulation scheme and a CDMA scheme, has been drawing attention. The OFDM-CDMA scheme is broadly categorized into a time domain spreading scheme and a frequency domain spreading scheme. Herein, the frequency domain spreading scheme is explained.
FIG. 1 is a schematic diagram illustrating the state of digital symbols before modulation processing; whereas FIG. 2 is a schematic diagram illustrating the layout of respective chips after modulation processing in the frequency domain spreading scheme. According to the frequency domain spreading scheme, each one symbol of N digital symbols which make up a serial data sequence (FIG. 1) is multiplied by a spreading code having a spreading factor of M. After spreading, M chips in parallel are subjected to IFFT processing sequentially on a symbol-by-symbol basis. As its result, N OFDM symbols for M sub-carriers are generated. That is, in the frequency domain spreading scheme, chips after spreading are aligned along the direction of the frequency axis (FIG. 2). In other words, the chips after spreading are placed on different sub-carriers respectively.
Here, if it is assumed that one digital symbol before modulation processing occupies a radio resource of a time width T and a frequency band width B (FIG. 1), it follows that, after the modulation processing, one chip occupies a time width of N×T and a frequency band width of B/N. Therefore, the area occupied in a time-frequency domain per one digital symbol becomes M×T×B after the modulation processing, which is M times of the area occupied by the one digital symbol before the modulation processing.
Herein, if it is assumed that the number of digital symbols N=8, and the spreading factor of M=8, are taken as an example, the signal pattern of OFDM symbols generated according to the frequency domain spreading scheme would be as illustrated in FIG. 3. As shown in this drawing, in the frequency domain spreading scheme, eight OFDM symbols are sequentially generated from t0 through t7, each corresponding to its counterpart of eight digital symbols differentiated from each other with different black/white shades and patterns on a time axis. During such a generating process, eight chips for each digital symbol are allocated to different sub-carriers f1˜f8 respectively.
By combining the OFDM modulation scheme and the CDMA modulation scheme as described above, it is possible to achieve an effective reuse, or to produce an effective statistical multiplexing effect. In addition to that, it is possible to realize a high-speed data transmission which is faster than under a single-carrier CDMA transmission. It is noted that, the “reuse” means that an identical frequency is made usable both in adjacent cells. Also note that, the “statistical multiplexing effect” means such an efficiency that a greater number of user signals are accommodated in comparison with under consecutive transmission, where such accommodation is made possible in conditions where timings at which a user has some data to transmit and timings at which the user does not have any data to transmit occur randomly in varying occurrences from user to user, achieved by the reduction of energy during time periods in which both communications parties do not transmit data.
By the way, recently, there have been demands for real-time transmission of large-capacity data such as moving pictures, etc. In order to realize such transmission, it is necessary to transmit data in a very high transmission rate by using a limited range of frequency bands.
Though it is true that the OFDM-CDMA scheme offers a high-quality data transmission with a relatively high transmission rate, faster communications is demanded as described above.